1. Field of the Invention
The present invention relates to a method and an apparatus for measuring an optical transfer characteristic, each of which is capable of measuring an optical transfer characteristic of an optical device in a wide optical frequency bandwidth with high resolution, on optical frequency axis or on optical wavelength axis.
2. Description of the Related Art
In case of measuring an optical transfer characteristic (characteristic of amplitude, dispersion, delay, phase, gain or the like) of an optical device such as an optical filter, an optical fiber, an optical transmission system or the like, there is currently used an apparatus for measuring an optical transfer characteristic (an optical transfer characteristic measuring apparatus) having been constructed, for example, as shown in FIG. 4. The optical transfer characteristic measuring apparatus includes two light sources one of which is a variable wavelength sweep type light source 10 for measurement (hereinafter referred to as variable wavelength sweep type measurement light source or measurement light source), and the other of which is a variable wavelength light source 20 for reference (hereinafter referred to as variable wavelength reference light source or reference light source). In the variable wavelength sweep type measurement light source 10, the wavelength of an optical or light signal generated thereby can be switched stepwise and also the optical frequency of the optical signal can be swept in a predetermined frequency range. In the variable wavelength reference light source 20, the wavelength of an optical or light signal generated thereby can be switched stepwise, but the optical frequency of the optical signal cannot be swept.
An output end of the variable wavelength sweep type measurement light source 10 is optically coupled to an input end of an optical device whose optical transfer characteristic is to be measured (optical device under measurement) DUT via a first optical coupler 30. In the illustrated example, the first optical coupler 30 has four ports, i.e., the first through the fourth ports 301, 302, 303 and 304. The first and the second ports 301 and 302 are used as input ports, respectively, and the third and the fourth ports 303 and 304 are used as output ports, respectively. The first port (the first input port) 301 is terminated in reflectionless termination, the second port (the second input port) 302 is coupled to the output end of the variable wavelength sweep type measurement light source 10 via an optical fiber, the third port (the first output port) 303 is coupled to an input end of the optical device under test DUT via an optical fiber, and the fourth port (the second output port) 304 is coupled to the second port of a second optical coupler 31 which will be discussed later. As a result, an optical signal generated from the measurement light source 10 is optically branched into two, one of which is supplied to the input end of the optical device under measurement DUT and the other of which is supplied to the second port of the second optical coupler 31.
The output end of the variable wavelength reference light source 20 is coupled to the second optical coupler 31 via an optical fiber. The second optical coupler 31 also has four ports, i.e., the first through the fourth ports 311, 312, 313 and 314. The first and the second ports 311 and 312 are used as input ports, respectively, and the third and the fourth ports 313 and 314 are used as output ports, respectively. The first port (the first input port) 311 is coupled to the output end of the variable wavelength reference light source 20 via an optical fiber, the second port (the second input port) 312 is coupled to the second output port 304 of the first optical coupler 30 as mentioned above, the third port (the first output port) 313 is terminated in reflectionless termination, and the fourth port (the second output port) 314 is coupled to an input end of a light receiving device 21 via an optical fiber. As a result, an optical signal generated from the reference light source 20 is inputted to the second optical coupler 31 via its first input port 311, and also an optical signal generated from the measurement light source 10 is inputted to the second optical coupler 31 via its second input port 312, so that the two light signals can be optically heterodyned.
Since the illustrated optical transfer characteristic measuring apparatus monitors the amount of frequency sweep (a frequency being swept) using an optical heterodyne detection, the optical signals generated from the variable wavelength sweep type measurement light source 10 and the variable wavelength reference light source 20 are first set to signals having the same frequency (the same reference wavelength), and then the frequency of the optical signal generated from the measurement light source 10 is swept within a predetermined frequency range so that both the optical signals are optically heterodyned in the second optical coupler 31. After that, an electrical signal having a frequency difference between both the optical signals, i.e., an electrical signal having an optical beat frequency is detected by the light receiving device 21 coupled to the second output port 314 of the second optical coupler 31.
As the light receiving device 21 is used, in this example, a photodetector for converting light into an electrical signal such as a photodiode, and the electrical signal having the optical beat frequency detected by the light receiving device 21 is supplied to a high frequency counter 50 after it has been amplified by an amplifier 22. Further, the light receiving device 21 and the amplifier 22 constitute a wide-band or broad-band optical receiver 29.
The high frequency counter 50 measures, based on the electrical signal having the optical beat frequency supplied from the wide-band optical receiver 29, the amount of sweep of the frequency of the optical signal generated from the variable wavelength sweep type measurement light source 10, and supplies the measured result to a measurement/analysis/display part 90. On the other hand, an optical or light signal for measurement (hereinafter referred to as optical measurement signal) inputted to the optical device under measurement DUT via the first optical coupler 30 is transmitted through the optical device under measurement DUT into the measurement/analysis/display part 90. The measurement/analysis/display part 90 carries out a measurement/analysis of the inputted optical signal, and displays the results of the measurement/analysis, if necessary. In addition, the measurement/analysis/display part 90 supplies a control signal to a variable wavelength light source control part 80, as the case may be.
Control terminals of the measurement light source 10 and the reference light source 20 are connected to output terminals of the variable wavelength light source control part 80, respectively, and the reference wavelengths of both the optical signals generated respectively from these light sources 10 and 20 are determined on the basis of wavelength setting signals supplied from the variable wavelength light source control part 80 to the control terminals of both the light sources, respectively. In addition, the variable wavelength light source control part 80 supplies a frequency sweep instruction or command signal to the control terminal of the variable wavelength sweep type measurement light source 10 at a predetermined timing, thereby to cause the frequency of the optical signal generated from the measurement light source 10 to be swept within the predetermined frequency range.
Incidentally, an optical frequency bandwidth that can be covered by one sweep is determined by the performance of the variable wavelength sweep type measurement light source 10, and this bandwidth is approximately several 10 GHz or so in the current technology level. Accordingly, the wavelength of the optical signal generated from the measurement light source 10 will be changed, by sweep of the frequency, from a set reference wavelength which is the minimum (or the maximum) wavelength within the sweep wavelength range corresponding to the optical frequency bandwidth of the several 10 GHz or so, until the maximum (or the minimum) wavelength within that sweep wavelength range.
The optical transfer characteristic of the optical device under measurement DUT is first measured at the set reference wavelength of an optical measurement signal generated from the variable wavelength sweep type measurement light source 10, and next the frequency of the optical measurement signal of the measurement light source 10 is swept within the optical frequency bandwidth of several 10 GHz or so, so that the optical transfer characteristic of the optical device under measurement DUT can be measured at the set reference wavelength as well as at each of measurement points subsequent to the set reference wavelength spaced a constant distance or interval apart, which are determined by the resolution of the measuring apparatus (in other words, at each of optical frequencies corresponding to these measurement points). Therefore, the variable wavelength sweep type measurement light source 10 can perform its sweep operation with sufficiently high resolution in respect of that optical frequency bandwidth of several 10 GHz or so.
Further, as the variable wavelength reference light source 20, it is preferable to use a high precision light source which is capable of generating an optical signal having more precisely controlled reference wavelength as compared with the reference wavelength of the optical measurement signal generated from the variable wavelength sweep type measurement light source 10.
The operation of the optical transfer characteristic measuring apparatus constructed as mentioned above will be described with reference to a flow chart shown in FIG. 5.
First, in step SP1, a wavelength setting signal is supplied to both of the light sources 10 and 20 from the variable wavelength light source control part 80, thereby to set the wavelengths of the optical signals to be generated respectively from these light sources 10 and 20 to a specified identical reference wavelength, for example, xcex0 so that optical signals each having this reference wavelength xcex0 are outputted from both light sources.
The optical signal of the reference wavelength xcex0 generated from the variable wavelength sweep type measurement light source 10 is inputted to the optical device under measurement DUT and the second optical coupler 31 via the first optical coupler 30. The optical signal inputted to the optical device under measurement DUT is transmitted through this optical device under measurement DUT to the measurement/analysis/display part 90 so that an optical transfer characteristic of the optical device under measurement DUT is measured. On the other hand, the optical signal inputted to the second optical coupler 31 does not optically interfere with the optical signal generated from the variable wavelength reference light source 20 because the wavelength of the optical signal inputted to the second optical coupler 31 is the same, at this point in time, as the wavelength xcex0 of the optical signal outputted from the variable wavelength reference light source 20. As a result, the difference frequency between both the optical signals is zero, and hence no detected output is generated from the wide-band optical receiver 29. Accordingly, the amount of sweep on the optical signal outputted from the high frequency counter 50 is zero.
Next, the process proceeds to step SP2 in which the variable wavelength light source control part 80 supplies a frequency sweep instruction signal to the variable wavelength sweep type measurement light source 10, thereby to sweep the frequency of the optical signal outputted from the variable wavelength sweep type measurement light source 10 within a predetermined frequency range (optical frequency bandwidth of several 10 GHz or so).
When the sweep of the optical frequency is started, a detected output is outputted from the wide-band optical receiver 29. Accordingly, in step SP3, the detected output from the wide-band optical receiver 29 is measured by the high frequency counter 50. Based on the measured result, the measurement/analysis/display part 90 measures the amount of sweep of the optical frequency. Next, in step SP4, a decision is rendered whether the amount of sweep of the optical frequency has reached a limit of the ending side of sweep or not, that is, whether the frequency of the optical signal has been swept up to the upper limit (or the lower limit) of the predetermined frequency range or not.
During the time period that the amount of sweep of the optical frequency has not reached that limit (during the time period that the decision indicates xe2x80x9cNOxe2x80x9d), in step SP5, the optical transfer characteristic of the optical device under measurement DUT is measured at each of measurement points spaced a constant distance or interval apart starting at the set reference wavelength xcex0 (measured at each of optical frequencies corresponding to these measuring points), the constant distance being determined by the resolution of the measuring apparatus. The measurement at each of the measuring points is performed by repeating the steps SP2, SP3, SP4 and SP5.
On the contrary, in the step SP4, when the amount of sweep of the optical frequency has reached the limit of the ending side of sweep (YES), the process proceeds to step SP6 in which the sweep of the optical frequency of the optical signal having the reference wavelength xcex0 generated from the variable wavelength sweep type measurement light source 10 has been ended.
Next, the process proceeds to step SP7 in which the measurement/analysis/display part 90 analyzes the optical transfer characteristics of the optical device under measurement DUT at the reference wavelength xcex0 as well as at the measurement points (at each of the optical frequencies) ranging from the reference wavelength xcex0 to the upper limit or the lower limit within the predetermined frequency range, and displays or outputs the results of the measurement/analysis, if necessary. By this process, the measurement of the optical transfer characteristic of the optical device under measurement DUT has been completed.
As mentioned above, in the optical transfer characteristic measuring apparatus and method currently used, there are prepared, in order to measure the optical transfer characteristics of the optical device under measurement with high resolution on optical frequency axis or on optical wavelength axis, two types of light sources, one being the variable wavelength sweep type measurement light source 10 and the other being the variable wavelength reference light source 20, and the optical transfer characteristics of the optical device under measurement are measured while the sweep frequency on the optical measurement signal generated from the variable wavelength sweep type measurement light source 10 is being monitored by use of the optical heterodyne detection in which the optical signal generated from the variable wavelength reference light source 20 is used as a reference light.
However, the frequency bandwidth of the variable wavelength sweep type measurement light source 10 that can be covered by one frequency sweep is that of several 10 GHz or so, and hence there is a limitation in the sweep frequency bandwidth. Since a measurable optical frequency bandwidth is determined by this limitation in the sweep frequency bandwidth, the current optical transfer characteristic measuring apparatus and method cannot measure an optical transfer characteristic in wide optical frequency bandwidth with high resolution.
That is, as discussed above, the sweep frequency bandwidth of the optical signal generated from the variable wavelength sweep type measurement light source 10 is a bandwidth of only several 10 GHz or so. On the contrary, an operating frequency bandwidth of an optical device used in, for example, optical communications is much wider than the frequency bandwidth of several 10 GHz or so. Accordingly, there is a serious drawback that if an optical transfer characteristic of such optical device is measured by one frequency sweep of the variable wavelength sweep type measurement light source 10 and the optical transfer characteristic thereof is evaluated, the measured and evaluated results are unreliable. In other words, it is desirable to measure an optical transfer characteristic of an optical device on optical frequency axis or on optical wavelength axis in a wide sweep frequency bandwidth comparable to the wide frequency bandwidth of the optical device.
Moreover, in the technology of wavelength division multiplexing transmission (WDM) in recent years, there is a tendency that a distance between wavelengths is becoming more and more narrow. Therefore, the resolution on optical frequency axis must sufficiently be ensured. In addition, in case of performing a computation or operation such as a differential or the like and/or an analysis on the optical frequency axis, if the sweep frequency bandwidth is narrow, the amount of data in one time operation or computation (frequency bandwidth) determined at the time of the operation becomes discontinuous in the starting area or the ending area of the optical frequency axis. As a result, an operation or computation process error (measurement error) occurs in the starting area or the ending area of the optical frequency axis.
It is an object of the present invention to provide a method for measuring an optical transfer characteristic which is capable of measuring an optical transfer characteristic of an optical device on optical frequency axis or on optical wavelength axis in wide optical frequency bandwidth with high resolution.
It is another object of the present invention to provide an apparatus for measuring an optical transfer characteristic which uses the aforesaid optical transfer characteristic measuring method.
In order to accomplish the above objects, in a first aspect of the present invention, there is provided a method of measuring an optical transfer characteristic comprising the steps of: sequentially shifting an optical frequency band such that any discontinuous point in frequency does not occur, performing a sweep of an optical frequency over each of the respective optical frequency bands, and measuring an optical transfer characteristic of an optical device under measurement a plurality of times; and concatenating the measured data of the plurality of times on optical frequency axis.
The optical frequency bands sequentially shifted have an overlapped portion between two adjacent optical frequency bands, the overlapped portion being composed of a frequency band portion in the sweep ending side of one optical frequency band which is swept earlier in time and a frequency band portion in the sweep starting side of the other optical frequency band which is swept next time.
In a preferred embodiment, the sweep of the other optical frequency band which is swept next time is started at an optical frequency in the proximity of the sweep ending side of the one optical frequency band which is swept earlier in time.
In a second aspect of the present invention, there is provided a method of measuring an optical transfer characteristic comprising the steps of: (A) generating an optical signal having a specified first wavelength from a variable wavelength sweep type light source which is capable of switching stepwise the wavelength of an optical signal generated thereby and sweeping the optical frequency of the optical signal in a predetermined frequency range; (B) measuring an optical transfer characteristic of an optical device under measurement at the specified first wavelength; (C) sweeping the optical frequency of the optical signal having the specified first wavelength; (D) measuring the optical transfer characteristic of the optical device under measurement at each of the respective wavelengths corresponding to a predetermined plurality of optical frequencies during the sweep of the optical frequency of the optical signal; (E) ending the sweep of the optical frequency of the optical signal when the sweep optical frequency of the optical signal has reached a frequency corresponding to a specified second wavelength to be generated next time from said variable wavelength sweep type light source; (F) measuring the optical transfer characteristic of the optical device under measurement at the specified second wavelength to be generated next time; (G) sweeping the optical frequency of the optical signal having the specified second wavelength; (H) measuring the optical transfer characteristic of the optical device under measurement at each of the respective wavelengths corresponding to a predetermined plurality of optical frequencies during the sweep of the optical frequency of the optical signal; (I) ending the sweep of the optical frequency of the optical signal when the sweep optical frequency of the optical signal has reached a frequency corresponding to a specified third wavelength to be generated next time from said variable wavelength sweep type light source; (J) repeating the steps (F) to (I) with regard to each of specified wavelengths to be generated from the variable wavelength sweep type light source until the specified wavelengths to be generated therefrom have been completed; and (K) concatenating, on optical frequency axis or optical wavelength axis, the measured results of the optical transfer characteristic regarding all of the specified wavelengths generated from the variable wavelength sweep type light source.
A distance between the plurality of specified wavelengths generated from the variable wavelength sweep type light source and switched stepwise is chosen such that it is shorter than a distance corresponding to the sweep frequency bandwidth of the optical signal.
In a preferred embodiment, by sweep of the optical frequency of the optical signal, a wavelength range is swept, which starts at a predetermined wavelength in the side of a specified wavelength generated earlier in time than the associated specified wavelength, and extends therefrom through this associated specified wavelength to a specified wavelength generated next time.
By sweep of the optical frequency of the optical signal, a wavelength range may be swept, which starts at the associated specified wavelength, and extends therefrom through a specified wavelength generated next time to a predetermined wavelength beyond this next generated specified wavelength.
Alternatively, by sweep of the optical frequency of the optical signal, a wavelength range may be swept, which starts at a predetermined wavelength in the side of a specified wavelength generated earlier in time than the associated specified wavelength, and extends therefrom through this associated specified wavelength and a specified wavelength generated next time to a predetermined wavelength beyond this next generated specified wavelength.
The amount of sweep of the optical frequency of the optical signal is monitored by an optical heterodyne detection of an optical signal generated from the variable wavelength sweep type light source.
In a third aspect of the present invention, there is provided an apparatus for measuring an optical transfer characteristic comprising: a variable wavelength sweep type light source capable of switching stepwise the wavelength of an optical signal generated thereby and sweeping the optical frequency of the optical signal in a predetermined frequency range; light source control means for setting the wavelength of an optical signal generated from said variable wavelength sweep type light source to a specified wavelength as well as causing a sweep of the optical frequency of the generated optical signal having the specified wavelength over said predetermined frequency range to be carried out; first optical coupling means for inputting the optical signal generated from the variable wavelength sweep type light source into an optical device under measurement; measuring means optically coupled to said optical device under measurement for receiving an optical signal outputted from the optical device under measurement to measure an optical transfer characteristic of the optical device under measurement; counting means for counting the number of occurrence times of optical signals generated from the variable wavelength sweep type light source, the wavelength of each of said optical signals having been set to a specified wavelength; and means for ending the measurement of the optical transfer characteristic of the optical device under measurement, in response to a count signal outputted from said counting means when the number of occurrence times of optical signals each having a set specified wavelength has reached a preset number of times; and wherein said light source control means causing the variable wavelength sweep type light source to generate optical signals having their wavelengths sequentially switched and set to corresponding specified wavelengths as well as causing a sweep of the optical frequency of each of the generated optical signals having the corresponding specified wavelengths over said predetermined frequency range to be carried out, until the number of occurrence times of the optical signals each having a set specified wavelength reaches the preset number of times; and said measuring means concatenating, on optical frequency axis or optical wavelength axis, the measured results of the optical transfer characteristic regarding all of the specified wavelengths after the measurement of the optical transfer characteristic has been completed, and analyzing/displaying it.
In a preferred embodiment, the measuring apparatus further comprises: a variable wavelength reference light source capable of switching stepwise the wavelength of an optical signal generated thereby and incapable of sweeping the optical frequency of the optical signal; second optical coupling means into which are inputted an optical signal generated from said variable wavelength reference light source and an optical signal generated from said variable wavelength sweep type light source, and outputting an optical signal which has been optically heterodyned; and optical detecting means for detecting the optical signal outputted from said second optical coupling means to detect an electrical signal having an optical beat frequency; and wherein the wavelength of an optical signal generated from said variable wavelength reference light source is set to the same wavelength as the specified wavelength of an optical signal generated from the variable wavelength sweep type light source by the light source control means; said second optical coupling means optically heterodynes an optical signal of a specified wavelength generated from the variable wavelength reference light source and an optical signal. the wavelength of which is varied by a sweep of the optical frequency thereof, generated from the variable wavelength sweep type light source, and inputs the heterodyned optical signal into said optical detecting means; and the amount of sweep of the optical frequency is monitored on the basis of the electrical signal having an optical beat frequency detected by said optical detecting means.
In addition, the second optical coupling means optically heterodynes the optical signal generated from the variable wavelength sweep type light source and optically branched by the first optical coupling means, and the optical signal generated from the variable wavelength reference light source, and inputs the heterodyned optical signal into the optical detecting means.
Moreover, the measuring apparatus further includes: high frequency counting means for measuring the amount of sweep of the optical frequency on the basis of a detected output from the optical detecting means, and supplying the measured result to said measuring means; frequency-to-voltage converting means for converting a detected output from the optical detecting means into a voltage signal; a memory in which wavelength switching data are previously stored, the number of the wavelength switching data being equal to the number of specified wavelengths of optical signals generated from the variable wavelength sweep type light source, and the specified wavelengths being switched in one measurement; and voltage comparing means for comparing a voltage signal outputted from said frequency-to-voltage converting means with data read out of said memory; and wherein said counting means counts the number of occurrence times of optical signals generated from the variable wavelength sweep type light source, the wavelength of each of the optical signals having been set to a specified wavelength, by counting comparison result signals outputted from the voltage comparing means.